This invention relates to amplifier circuits and more particularly to integrated circuit amplifiers having a dual transconductance current path.
The required characteristics and features of integrated circuit amplifiers are evolving as new uses for integrated circuit amplifiers develop. One such use is in a new generation of portable electronic products such as the personal digital assistant (PDA), digital cellular phones, etc. that utilize a speaker phone feature, thus requiring a loud speaker. The smaller the impedance rating of the speaker, the louder it sounds for a given voltage drop across it. Since most of these portable products run off low power supplies (2.7 v-5 v), the voltage drop across the speaker is limited. Hence, low impedance speakers (4 to 32 ohms) are used to produce sounds of reasonable loudness out of them. These electronic products use integrated circuits to perform the signal processing and to interface with the external components such as speakers, microphones, etc.
In order to deliver such large power into the speaker, the integrated circuits that interface with this speaker require a power amplifier capable of delivering the necessary current into the speaker for the desired voltage drop across it. As the output of the amplifier is connected to the speaker, the sound is modulated according to the input to the amplifier. To generate large currents, the amplifier should have an output stage with very large transistor devices (generally current doubles with twice the device size). The use of large devices requires much silicon area and dc power (the power when driving 0 volts across the speaker) both of which are undesirable requirements. In addition to the loudness, the amplifier design also controls the sound quality of the speaker. Thus, the amplifier should be carefully designed not only to minimize the silicon area and the dc power consumption, but also to be linear so as to produce reasonable sound quality out of the speaker. Unfortunately, linearity generally degrades with low available dc power.
Class-AB amplifier design is a design methodology which gives the designer control over the dc power consumption of the driver. A side effect of this design philosophy is that the designer also gets control over the linearity of the amplifier, which generally degrades with low available dc power. Hence, the design of a suitable class-AB amplifier is nontrivial and challenging because it is desirable to optimize good sound quality, low power consumption, and low silicon consumption.